This invention relates to an image forming apparatus using a fixing device which passes a piece of paper (may hereinafter referred to as paper) bearing an unfixed toner image through the nip between a pair of heated rollers to heat and fuse an unfixed toner on the paper, thereby fixing the toner image onto the paper. More specifically, the invention relates to an image forming apparatus using a fixing device which controls energization of heating means based on the detected value of the roller surface temperature in a non-paper-passage region.
A conventional electrophotographic image forming apparatus uses a heat roller fixing system under which paper bearing an unfixed toner image is passed through the nip between at least a pair of rollers heated by heating means incorporated in a fixing roller which makes contact with an unfixed toner on the paper, among nip-forming roller pairs, whereby the toner on the paper is fixed. Under this heat roller fixing system, the surface temperature of the fixing roller needs to be heated to a temperature high enough to heat the toner on the paper, softening and melting the toner for fixing onto the paper. In order that the toner is fully softened in a short time during which the paper passes through the roller nip, the fixing roller is generally maintained at 140 to 210xc2x0 C., a temperature range several tons of degrees higher than the softening temperature of the binder resin contained in the toner. Heating means, such as a halogen heater, is incorporated within the fixing roller to heat the fixing roller. In order to maintain the surface temperature of the fixing roller uniformly regardless of the size of paper, a plurality of halogen heaters different from each other in heat distribution in the axial direction of the fixing roller are often incorporated in the fixing roller.
It is common practice to combine a middle region-emphasized halogen heater for heating the middle region of the fixing roller emphatically, and an opposite end region-emphasized halogen heater for heating the opposite end regions of the fixing roller emphatically. In the case of paper with a maximum width based on the axial direction of the fixing roller, the combined use of these heaters adjusts the entire paper-passage region of the fixing roller at a uniform temperature. The temperature of the fixing roller is set such that the temperature distribution during paper passage is uniform in the axial direction of the fixing roller. Thus, during a warm-up when no paper is passed, the middle region of the fixing roller is always at a high temperature, because of heat conduction to the non-paper-passage region or heat dissipation from the opposite end portions of the fixing roller. To provide a reference for heating control which maintains the temperature of the fixing roller at a constant fixing temperature, temperature detection means, such as a thermistor, for detecting the surface temperature of the fixing roller is brought into contact with a suitable position of the surface of the fixing roller. A thermistor has so far been mounted mostly so as to contact the surface of the fixing roller over which paper is actually passed. In this case, upon rubbing with the thermistor, the surface of the fixing roller is damaged, so that a paper release effect is impaired, diminishing the component life. Thus, the thermistor is mounted in the non-paper-passage region in an increasing number of embodiments.
However, the halogen heater is provided so as to heat the paper-passage region mainly. Thus, the thermistor in the non-paper-passage region is heated later than the paper-passage region. During a warm-up after the power is turned on, heating is continued, without passage of paper. At a time when the thermistor detects the fixing temperature, therefore, the surface temperature in the paper-passage region of the fixing roller rises excessively, posing the problems of a hot offset and a waste of power. To avoid these problems, the temperature in the middle region of the fixing roller may be kept down. In this case, the temperature in the opposite-end regions of the fixing roller may fail to reach the fixing temperature, causing a failure in fixing.
An object of the present invention is to provide an image forming apparatus equipped with a fixing device in which temperature detection means for the surface of a fixing roller is provided in a non-paper-passage region, the image forming apparatus being capable of preventing an excessive rise in the surface temperature of the middle region of the fixing roller during a warm-up.
Another object of the present invention is to provide an image forming apparatus equipped with a fixing device in which temperature detection means for the surface of a fixing roller is provided in a non-paper-passage region, the image forming apparatus being capable of preventing a hot offset in the middle region of the fixing roller and a failure in fixing in opposite-end regions of the fixing roller.
Yet another object of the present invention is to provide an image forming apparatus equipped with a fixing device in which a plurality of heating means are incorporated in a fixing roller and temperature detection means for the surface of the fixing roller is provided in a non-paper-passage region, the image forming apparatus being capable of heating the fixing roller such that the surface temperature of the fixing roller during a warm-up is uniform in the axial direction.
According to the present invention, there is provided an image forming apparatus comprising: a fixing device including a fixing roller, a pressure roller pressed into contact with the fixing roller, and temperature detection means in contact with a non-paper-passage region on the surface of the fixing roller, the fixing roller incorporating first heating means whose heat distribution based on the axial direction of the fixing roller is set such that heat in a middle region thereof is higher than heat in opposite-end regions thereof, and second heating means whose heat distribution based on the axial direction of the fixing roller is set such that heat in opposite-end regions thereof is higher than heat in a middle region thereof; and a controller for controlling power supply to the first heating means and the second heating means, and wherein the controller starts power supply to the first heating means and the second heating means to start warm-up of the fixing device, and cuts off power supply to the first heating means for a predetermined period of time during the warm-up.
Preferably, the controller cuts off power supply to the first heating means for the predetermined period of time at a time when the temperature of the non-paper-passage region detected by the temperature detection means reaches a predetermined value during the warm-up.
Preferably, the controller cuts off power supply to the first heating means for the predetermined period of time during the warm-up, then restores power supply to the first heating means, and continues power supply to the first heating means and the second heating means until a predetermined time when the warm-up is completed.
Preferably, ambient temperature detection means is provided, and the controller sets the duration of the warm-up based on the ambient temperature detected by the ambient temperature detection means.
Preferably, the controller cuts off power supply to the first heating means for the predetermined period of time during the warm-up, then restores power supply to the first heating means, and at a time when the temperature of the non-paper-passage region detected by the temperature detection means reaches a predetermined value, cuts off power supply to the first heating means and the second heating means, thereby completing the warm-up.